Just last week, we started hearing rumors of Nokia Lumia 920 firmware update for ATT network, thanks to folks at Navifirm leaking it. This confirmed us that the update was round the corner. The new update (1308) promises to make the camera quality better than before and brings new customizable options for adjusting the display quality.
So expect better white balance and improvement in image sharpness soon. So in a week or two, start manually checking for update on your ATT phone.